In a position detecting device which detects a piston rod based on the time elapsed from a point at which an ultrasonic wave is emitted from an ultrasonic wave sensor toward a lower end surface of the piston rod reciprocatingly moved linearly by supply and discharge of a working oil with respect to a hydraulic cylinder to a point at which the ultrasonic wave reflected by the lower end surface of the piston rod is received by the ultrasonic wave sensor, if the working oil's temperature is detected by a temperature sensor, then a microcomputer sets the strength of the ultrasonic wave emitted from the ultrasonic wave sensor in accordance with the temperature of the working oil so as to accurately detect the position of the piston.